In electric power transfer, when frequency conversion is to be performed on an alternating current, in a frequency converter, an alternating current is once converted into a direct current by a smoothing circuit employing a diode, an inductor, a capacitor, and the like. Then, in the frequency converter, switching is performed on the converted direct current by a semiconductor switching element so that conversion into an alternating current of desired alternating-current frequency is performed anew. For example, the frequency converter performs frequency conversion between a 50-Hz alternating current in eastern Japan and a 60-Hz alternating current in western Japan, frequency conversion between an alternating current of 400-Hz power generation frequency in the inside of an aircraft and a 50- to 60-Hz alternating current from an outlet, or the like. As the semiconductor switching element, a thyristor, a transistor, an FET, an IGBT, an SIT, or the like is employed. Further, another technique is also known that is referred to as a cycloconverter in which frequency conversion of an alternating current is performed directly by high-frequency switching. However, also in this technique, a semiconductor switching element is employed similarly.